ACTG 254 is a comparative study of two drug combinations (atovaquone/azithromycin vs. trimethoprim/sulfamethoxazole) for the prevention of serious bacterial infections in HIV-infected children. The lower age limit for enrollment into this study was lowered to 3 months (from 2 years) in March 1997. On February 12, 1999, this study will close to accrual of patients older than 19 months of age. Treatment and data collection continue in all age cohorts. Preliminary data from a pharmacokinetic substudy suggest that there is no pharmacokinetic interaction between atovaquone and azithromycin. No preliminary efficacy data are available from this study.